The first step is to increase the size of the main data-partition to fill out the empty space at the end of the partition table.

Luckily the partition in question is at the end of the table.

Don't make the mistake I made and use GParted for this. It will break your system and you have to write the SD image again.

If you haven't installed them on your system, install f2fs-tools version >= 1.15 and gdisk.

f2fs-tools 1.14 (and maybe older versions as well) will throw some wrongful error messages depending on the size of the SD card.

Some/Most/All commands in this guide need to be run with superuser rights.